Using for example the VoIP transmission method, it has become possible to provide for the transmission of a telephone signal, which is conventionally transmitted over an analog signal connection to a telephone network, e.g. a public-switched telephone network, via a broadband-communication link to a so-called IP network (Internet Protocol network). In this case, the connection from a communication device which is located at a subscriber site to the IP network typically involves a connection from the subscriber site to a central office according to a DSL transmission method or a cable-modem connection from the subscriber site to an access point of the IP network. In the central office usually a high-speed connection to the IP network is provided.
For providing a telephone service via the broadband-communication link, a service provider provides a telephone number which is assigned to the communication device located at the subscriber site. The VoIP transmission, i.e. the telephone signal, is received in the IP network and forwarded to another communication device and vice versa. The other communication device may be either directly connected to the IP network via a broadband-communication link as described above or may be connected to a conventional telephone network via an analog signal connection. In the latter case, the transmission of the telephone signal from the IP network to the telephone network is accomplished via a so-called VoIP gateway connecting the IP network and the telephone network.
Conventional telephone devices which are connected to the central office via an analog signal connection typically receive electrical power from the central office via the same physical line which carries the analog signal connection. Therefore, a conventional telephone device is usually also operable in case of a power failure at the subscriber site. However, communication devices for transmitting the telephone signal via a broadband-communication link are usually supplied with electrical power using a local power source at the subscriber site. Therefore, these communication devices are usually not operable in case of a power failure at the subscriber site or are only operable for a limited period of time by using a back-up power source, e.g. an emergency battery. Moreover, the transmission of the telephone signal using the broadband-communication link requires a large amount of electrical power as compared to the transmission of the telephone signal using an analog signal connection. Consequently, the back-up power source will soon be exhausted.
Therefore, in order to provide a telephone service also in the event of a power failure a communication device can be reconfigured in such a manner that an alternative communication link, e.g. an analog signal connection to a telephone network, is provided in case of a power failure at the subscriber site. This allows for maintaining the telephone service without excessive requirements of electrical power.
In the above approach of reconfiguring a communication device in case of a power failure there exist, however, problems in that an ongoing transmission of the telephone signal, i.e. an ongoing call, will be dropped or interrupted if the power failure occurs during the transmission of the telephone signal via the broadband-communication link. This dropping of an ongoing call is not desirable for a user of the communication device and, moreover, is considered to be unacceptable by some service providers, because the ongoing transmission of the telephone signal may constitute an emergency call.
Interrupting the transmission or dropping the call could be prevented by maintaining the transmission on the broadband-communication link for as long as the call continues. But this has the disadvantage of using a lot of electrical power, so that the back-up power source will be exhausted by keeping components for maintaining the broadband-communication link active, as these components typically require a large amount of electrical power.